The invention relates to a method and a device for measuring electric currents by means of a current transformer. In particular, the invention relates to a method and a device for measuring the electric differential currents which, in addition to the alternating component, also contain a direct current component, and are thus mixed currents. The measurement of differential currents and/or residual currents is a major concern in the field of safety engineering of industrial plants and electrical equipment, in particular for the protection of people against electric shocks, but also for the protection of machinery, industrial equipment and electrical equipment against malfunction and damages due to unwanted or faulty current flows. An example of a possible occurrence of such unwanted current flows is so-called ground faults. In the case of a ground fault, an electric conductor forms an unintentional electrical connection to the ground potential, that is, there is a low electrical resistance between conductor and ground potential. Electric current, residual current or differential current flows across this resistance. This presents a problem and a major hazard when thresholds are exceeded during the operation of electrical equipment.
To prevent such a hazard, the existence and the magnitude of differential or residual currents within the device are determined. In the case of a residual current, one distinguishes between the smooth DC residual current and the pulsating DC residual current as well as the AC residual current. Inductive current transformers or summation transformers are used to detect alternating residual current. An inductive current transformer or summation transformer generally comprises a ferromagnetic transformer core, on which a transformer coil, for example a coated copper wire, is disposed as a secondary winding. To detect, for example, a differential current, the supply line and return line of the circuits of the electrical device in question are combined and guided together in parallel through a current transformer. The electrical supply line and the return line together form the primary winding of the current transformer, whereby the primary winding does not necessarily have to wrap around the ferromagnetic transformer core repeatedly or once, but can generally consist of conductors fitted in a straight manner through the middle of the transformer core. To be more specific, the simply pushed-in conductors are exactly one winding that closes at a great distance from the transformer core.
When an electric device functions properly, i.e., when the electrical resistance between electrical conductors and the ground potential is sufficiently great, the sum of the currents in the electrical supply and return lines through the current transformer will cancel out and the current transformer will not output any signal. In the presence of undesirable current flows in the electrical device, however, a finite differential current will be measured.
As with transformers, inductive current transformers are only sensitive to alternating electric currents, whereby, without further measures, a direct current does not output a suitable signal. In practice, however, since the measurement of the direct current component of the differential current is of primary interest, for example, due to increased risk of direct current to humans, different devices comprising current transformers have been proposed, which are also capable of measuring the direct current component of differential currents.
From DE102005028881 B4, a residual current analyzer for detecting residual currents and a device with inductive summation current transformers for detecting AC residual current and pulsating direct current is known, in which by means of the filtering and splitting of the detected differential current signal into low-frequency and high-frequency sub-signals and their analysis, residual currents can be detected which can usually not be detected with such inductive summation current transformers. Furthermore, a calibration of the residual current analyzer through feeding a specifically adjustable and known residual current through an additional winding on the summation current transformer is proposed. However, the measured direct current is not smooth.
DE 102 37 342 A1 discloses a method and a device for monitoring residual current in alternating current networks, in which current sensors are used to detect the currents and digitize them; subsequently, the summation current is calculated. In dependence on the phase position for the voltage, the summation current is divided into the active and reactive current components, enabling a frequency-dependent weighting of the sum of currents which corresponds to the residual alternating current. Residual direct currents can be detected only when using direct current sensors. To measure a relatively small differential current, for example 10 A+(−9.99 A)=10 mA, very accurate and expensive current sensors are required in order to make a practical implementation possible at all.
Furthermore, methods for the measurement of mixed currents and especially smooth DC residual currents using inductive current transformers are known, which utilize the non-linearity between magnetic flux density B and field strength H according to the hysteresis and/or magnetization curve B(H) of the ferromagnetic transformer core. More particularly, the fact is utilized that the transformer core at an increasing flow of current through the primary conductor thus reaches saturation, thereby flattening the increase of the magnetization curve B(H) towards higher primary currents and thus resulting in a dependency of the permeability from the current flowing through the primary conductor. In terms of measurement technology, it therefore determines the instantaneous value of the existing differential current, the point on the magnetization curve of the transformer core that is assumed, whereby the rise dB/dH of the point occupied on the magnetization curve determines the differential inductance of the coil in the secondary circuit of the current transformer, which is then measured by means of suitable circuits.
In DE19943802 and/or EP1212821, the principle of controlled inductance is used. Changes in coil inductance are thereby detected on the basis of the detuning of a resonant circuit. In DE19943802, the principle of the transducer circuit is used, whereby the differential current acts as a control current of the transducer. When a DC residual current exists, the iron core magnetization shifts and changes the coil inductance. Changes in coil inductance are thereby detected on the basis of the detuning of a resonant circuit. Further examples of the application of the principle of controlled inductance are described in DE3642393 A1 and DE3543985 A1.
It is also known to use a coil as an applied inverting and frequency-determining component of a multivibrator. This creates a rectangular alternating voltage on the coil such that the ferromagnetic transformer core always oscillates back and forth between its two saturation magnetic fluxes. A magnetization current thereby flows through the coil. With a suitable form of the magnetization curve of the material of the transformer core it can be achieved in such a device in that the magnetic flux field traversed by the transformer core is almost independent of the instantaneous value of the differential current. It follows that each magnetic voltage generated by the differential current over the transformer core is compensated by a magnetic offset voltage generated by coil. The magnetization current of the multivibrator is superimposed on a counter current proportional to the differential current, which is then measured by means of suitable circuits.
DE19826410 A1 shows the basic circuit of an all-current sensitive differential current sensor, whereby the multivibrator is realized with two applied coils.
In EP1267467 A2, a modulating oscillation circuit is described which comprises a multivibrator with a coil. Here, a resistor in the magnetization circuit brings about that the counter-current influences the pulse-width ratio of the generated rectangular alternating voltage. However, it is set out in EP1267467 A2 that in the described method through high-frequency differential current components a violation of the sampling theorem of Shannon can occur. According to that solution it is proposed that the magnetic voltage generated by the high-frequency differential current components in the transformer core, for example, of the multivibrator be compensated by an opposite magnetic voltage. The opposite voltage is generated by an additionally applied coil that is connected to an additional, inductively operating transformer via a high pass filter. Through this measure, aliasing effects between the high-frequency differential current components and the multivibrator frequency are avoided in the multivibrator.
DE 3 534 985 A1 and DE 3 543 948 B1 disclose a residual current circuit breaker for detecting universal current consisting of two summation current transformers. Hereby a summation current transformer transforms pulsating and alternating currents and a second summation current transformer detects direct current.
From DE29705030 a residual current circuit breaker for detecting universal current with a summation current transformer is known. Here, a summation current transformer with two separate evaluation circuits for pulsating and/or alternating and direct current are provided, which are operated by clocks or filters. In this case, the current transformer is either operated alternately over clock pulses with the evaluation circuits or connected simultaneously via filters with two evaluation circuits.
A disadvantage of the known solutions with inductive current transformers is that the measurement of the DC residual current is indirectly effected by evaluating the alternating current component, whereby changes in inductance are determined. This leads to complications in the evaluation and the connection circuit. In addition, increased requirements in terms of coil winding and core materials are to be met. Moreover, the amount of wiring for the described current transformer for detecting mixed currents is often higher than for detecting alternating currents. In some embodiments, part of the measurement electronics is housed in the transformer housing, which requires the power supply. Other versions operate with two coils on the transformer core and require a four-wire connection.
This all limits the possibility to retrofit already available or already built-in current transformers that are used to detect alternating current to detect mixed current.
Furthermore, a measurement of mixed currents in the ampere range can alternatively also be realized with the aid of Hall current transformers, wherein the Hall element is located in the air gap of a ferromagnetic iron core. A current in the primary conductor surrounded by a core leads to a magnetic flux through the Hall element and an evaluable Hall voltage. Often Hall current transformers work according to the principle of compensation. For this, a coil is disposed on the transformer core. This is controlled by a control circuit connected with the Hall element such that the magnetic flux through the Hall element is always equal to zero. Each magnetic voltage generated by the enclosed primary conductor is compensated by a counter voltage. The necessary countercurrent needed for this is proportional to the current of the primary conductor and is the output signal of such transformers.
Hall current transformers are not suitable for measuring differential currents, which can lie in the 10 mA range. The object of the invention is to provide a suitable measuring method for mixed currents and a corresponding measuring device that works with the same current transformers that are commonly used for measuring AC residual currents.